1. Field of the Invention
The present invention relates to a pattern defect detection device for detecting defects and foreign matter in patterns of a regular arrangement such as in a patterned semiconductor integrated circuit, and more particularly, to improvement of a spatial frequency filter used in such a pattern defect detection device.
2. Description of the Background Art
FIG. 1 is a block diagram schematically showing a pattern defect detection device disclosed in Japanese Patent Laying-Open No. 63-205775. This pattern defect detection device includes a light source 1 of a laser oscillator and the like for generating coherent light, a collimator 2 to expand and collimate the coherent light emitted from source 1 to become parallel, a half mirror 3, a stage 5 for supporting a specimen 4 having a patterned surface, a lens 6 for focusing light diffracted by the pattern on the surface of specimen 4, a half mirror 7, and a spatial frequency filter 8 having a pattern of a plurality of black spots, supported in a holder 8a at the back focal plane of lens 6 for transmitting only diffracted light corresponding to the defects of the pattern and blocking diffracted light of a proper pattern.
The pattern defect detection device of FIG. 1 also includes a defect detection TV camera 9 (defection signal detection portion) for detecting diffracted light passing through the transparent portion of spatial frequency filter 8, i.e. through the portion other than the black spots, a first signal processor 10 for detecting the defect position according to the diffracted light detected by defect detection TV camera 9, a TV monitor 11 for displaying the defect, a position detection TV camera 12 for detecting the position of the diffraction pattern according to reflected light from half mirror 7, a second signal processor 13 for detecting the position of the diffraction pattern according to the reflected light detected by position detection TV camera 12, and a controller 14 to calculate the position offset distance from the proper position of the diffraction pattern according to the defect position and the diffraction pattern position detected by first and second signal processors 10 and 13 for sending a correction command to tilt-azimuth angle adjustment mechanisms 15, 16 and to a rotation angle adjustment mechanism 28.
The operation of the device of FIG. 1 will be explained hereinafter. The coherent light emitted from laser oscillator 1 is collimated into a parallel ray by collimator 2 and reflected by half mirror 3 to be directed to the pattern on specimen 4 such as a semiconductor integrated circuit wafer. Diffracted light from specimen 4 passes half mirror 3 to be converged by lens 6 and divided into two by half mirror 7. The diffracted light passing half mirror 7 reaches to spatial frequency filter 8, whereas the diffracted light reflected by half mirror 7 enters position detection TV camera 12.
At the time of defect detection, the diffracted light reaching to spatial frequency filter 8 through half mirror 7 has the component of proper pattern of specimen 4 removed by the pattern of black spots on spatial frequency filter 8, so that only the component of a defect is transmitted as a defection signal. This defection signal is received by defect detection TV camera 9 and detected by first signal processor 10. This defect is displayed on monitor 11.
During the defect detection, registration of the diffraction pattern represented by black spots on spatial frequency filter 8 and the pattern of the diffracted light of the proper pattern on specimen 4 must be carried out. More specifically, the diffracted light of half mirror 7 is observed by position detection TV camera 12, whereby the position of the diffracted light pattern is detected by second signal processor 13. The output signals of signal processors 10 and 13 are received by controller 14, whereby the position offset distance of the diffracted light pattern with respect to the position of the pattern of black spots on filter 8 is calculated from these output signals to provide an instruction to tilt-azimuth angle adjustment mechanisms 15, 16 and to rotation angle adjustment mechanism 17 to correct the deviation of the tilt-azimuth angle of the optical axis of the specimen 4 surface and the rotation angle. Thus, a correct matching is carried out of the black spot pattern on spatial frequency filter 8 and the diffracted light pattern of proper pattern on specimen 4.
The above-described spatial frequency filter 8 is produced by exposing a photosensitive plate with a diffracted light pattern of a proper pattern at the back focal plane of lens 6. This filter 8 is then accurately positioned and fixed at the back focal plane of lens 6.
Since a conventional pattern defect detection device was implemented as described above, it was always necessary to accurately match the pattern of black spots on spatial frequency filter 8 and the diffracted light pattern by monitoring change in the angle of reflection of the wafer surface, i.e. the tilt-azimuth angle, to move stage 5 including tilt-azimuth angle adjustment mechanisms 15 and 16 during the defect detection.
The actual subject to be inspected, for example a semiconductor wafer, is subjected to various thermal treatments, wherein many have an undulating surface. The undulating surface of specimen 4 attributed to a great correction of the tilt-azimuth angle, with a possibility that this correction may not be completed within a predetermined time period. A great amount of correction required a long time period for this correction, leading to increase in the time required for the entire inspection. If defect detection was carried out when the correction of the tilt-azimuth angle was not thoroughly completed, there were some cases where the Fourier transform pattern of the black spot pattern on spatial frequency filter 8 and that of the pattern on specimen 4 do not match partially (or in some cases entirely). Light signal passing through the not-matching portion caused reduction in the ratio between the defection signal to be detected and the signal from the proper pattern which should not actually be detected, i.e. noise. This led to a problem that there was a limit in the minimum size of the defect that could be detected.
In specimen 4 having a complicated pattern such as in a semiconductor integrated circuit wafer where there is a pattern region including highly repetitive components such as memory cells, and a pattern region including few repetitive components that will reduce the diffracted light intensity to one order of magnitude lower, recording of the pattern region of the low repetition number could not be carried out sufficiently for creating a spatial frequency filter 8 just by a one time exposure on a photosensitive plate. A filter 8 not sufficiently recorded could not completely block the diffracted light pattern and passes weak remaining diffracted light pattern. On the contrary, if the photosensitive plate is exposed until a spatial frequency filter 8 is obtained that has a spot of density that can completely block the diffracted light pattern from the pattern region of the low repetition number, the portion on the photosensitive plate recording diffracted light from the pattern region of the high repetition number will have excessive exposure energy applied corresponding to the right end portion of the density curve of FIG. 2.
Referring to the graph of FIG. 2, the abscissa shows exposure light energy (erg/cm.sup.2) having a wave length of 633 nm, and the ordinate represents the optical density D of a photosensitive plate exposed by that light energy. The optical density D is defined as D=log.sub.10 (I.sub.in /I.sub.out), where I.sub.in denotes intensity of incident light and I.sub.out denotes intensity of transmitted light. Over-exposure occurs in the portion where diffracted light from the pattern region having many repetitive components are recorded to generate halation. This halation uncontrollably expands beyond the spot portion to be recorded to form unnecessary screening portions, whereby the defect signal transmittance of spatial frequency filter 8 is reduced. There is also a possibility of light being transmitted through a portion which should actually be a screening pattern portion. This means that detection will be carried out where remaining diffracted light pattern exists, leading to a problem that the region that can be detected correctly is limited to specific portions, such as in memory cells.
When spatial frequency filter 8 is formed by a photosensitive plate such as a photosensitive material of silver halide emulsion, the density of the spots on filter 8 does not exceed a constant value, as shown in FIG. 2. More specifically, the light signal screening ability of spatial frequency filter 8 decreases the light intensity to only 1/1000-1/10000. This means that a proper pattern signal not blocked at the filter will pass therethrough to act as background noise, whereby the minimum size of the defection signal intensity that can be detected is inevitably determined. Therefore, a very small defect could not be detected that has a signal intensity that is less than 1/1000-1/10000 of the signal intensity from a proper pattern.